I'm 90% sure that that is his throne world, where we killed him.
In the mission "regicide" we killed his physical form, which is why he came off as weak compared to his near god-like raid form. A Hive throne world is what he makes of it. Crotas throne world was just weird. Oryxs is his dreadnought.
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As above, I believe that the walls of the ship are the limit to the throneworld, read the comment I made below to find my while reasoning.